Study on high-frequency dielectric anomaly in charge ordering

1)用腔微扰法测量了Pr_&lt；0.5&gt；MnO_&lt；3-δ&gt；0.5&gt；MnO_&lt；3-Pr_&lt；0.5&gt；在4.2K~270K温度范围内12T磁场下的电磁响应，以研究带电有序的高频介电反常现象。2)用浮区法生长单晶。在圆柱腔TE&lt；011&gt；模中，微波振幅随场的变化在CO跃迁附近有一个宽峰和一个尖峰。共振频率随场的变化在幅度尖峰处表现出极小值。这些被认为是与相变时电荷重新分布有关的介电反常。5)首次得到了由电磁响应估计的B-T相图。这与磁化测量的结果相似，这表明微波介电响应对于改变B-T相图的轮廓并不是至关重要的。6)谐振频率从绝缘体到金属相的变化与通常在金属-绝缘体相变中观察到的变化相反。这可能导致在高场区激活能为25 meV的微波电导率与半导体温度有关，在高场区，直流电阻表现出金属温度依赖性。7)通过磁阻测量对钴铁矿进行了额外的研究，其中包括钴离子的电荷分离。在Ca&lt；3-x&Gt；Y_xCo_4O_9体系中观察到了巨磁电阻效应，这证明了颗粒磁性薄膜中的电荷分离是由隧道磁阻效应来解释的。

1)Electromagnetic response of Pr_<0.5>Ca_<0.5>MnO_<3-δ> at 34 GHz was measured by a cavity perturbation method in the temperature range from 4.2 to 270 K under the magnetic field up to 12 T in order to investigate the high frequency dielectric anomaly with charge ordering.2)The single crystals were grown by a floating zone method.3)B-T phase diagram was determined from the magnetization and magnetoresistance measurements using a pulsed magnet.4)When the sample is placed at the electric field maximum E_<max> in the TE_<011> mode of cylindrical cavity, the microwave amplitude shows a broad peak and a sharp peak around the CO transition as a function of field. The resonant frequency shows a minimum at a sharp peak in amplitude as a function of field. These are considered as the dielectric anomalies related to a redistribution of charges at the transition.5)The B-T phase diagram estimated from the electromagnetic response is obtained for the first time. It is similar to that from the magnetization measurements, which suggests that the microwave dielectric response is not critically important to alter the profile of the B-T phase diagram.6)The shift of resonant frequency from an insulator to a metal phase shows an opposite change to that usually observed in a metal-insulator transition. This may lead to the semiconducting temperature dependence in microwave conductivity with activation energy of 25 meV at high field region, where the dc resistance shows metallic temperature dependence.7)Additional study was performed in a cobaltite by the magnetoresistance measurements with interest in the charge separation of Co ions. Colossal magnetoresistance was observed in Ca_<3-x>Y_xCo_4O_9 system, which evidences for the charge separation explained by a tunneling magnetoresistance effect as usually observed in granular magnetic films.